This invention relates to use of ambient noise to drive a resonant, passive acoustic jet having a nozzle directed, preferably essentially tangentially, into the flow of a fluid to energize the boundary layer thereof.
Fluid flow in the boundary layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the wall: essentially zero at the surface, raising to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the fluid times the square of its velocity. This near-wall, low-momentum fluid can be problematic for the case where the static pressure rises along the direction of the flow. For example, along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case in a diffuser, on the suction side of an airfoil such as a fan blade or an airplane wing, the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. If the pressure rise is sufficiently large, the momentum and energy of the fluid along the surface is consumed in overcoming this pressure rise, so that the fluid particles are finally brought to rest and then flow begins to break away from the wall, resulting in boundary layer separation. Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift) and dramatic decrease in system efficiency, due to conversion of flow energy into turbulence, and eventually into heat. It is known that boundary layer separation can be deterred by increasing the momentum flux of the fluid particles flowing near the surface. In the art, the deterrence or elimination of boundary layer separation is typically referred to as xe2x80x9cdelaying the onset of boundary layer separationxe2x80x9d.
The simplest and most common method for overcoming boundary layer separation includes small vortex generators, which may typically be tabs extending outwardly from the surface (such as the upper surface of an airplane wing), which shed an array of streamwise vortices along the surface. The vortices transport the low momentum particles (that are flowing near the surface upstream) away from the surface downstream, and transport the higher momentum particles (that are flowing at a distance from the surface upstream) toward the surface downstream, thereby improving the momentum flux of particles flowing near the surface in the boundary layer. This has the effect of deterring boundary layer separation at any given velocity and over a range of angle of attack (where the uncontrolled separation is downstream of the vortex generators). However, as is known, tab-type vortex generators create parasitic drag which limits the degree of boundary layer separation which can be efficiently/practically suppressed.
Another known approach employs continuous flow into or out of the boundary layer. A wall suction upstream of the boundary separation line (that is the line at which the onset of full boundary layer separation occurs across the surface of an airfoil or a diffuser) simply removes low momentum flux fluid particles from the flow adjacent to the surface, the void created thereby being filled by higher momentum flux particles drawn in from the flow further out from the surface. A similar approach is simply blowing high energy fluid tangentially in the downstream direction through a slot to directly energize the flow adjacent to the surface. Both of these flow techniques, however, require a source of vacuum or a source of pressure and internal piping from the source to the orifices at the surface. This greatly increases the cost, weight and complexity of any such system, and have not as yet been found to be sufficiently effective to merit further use, over a wide range of applications.
A relatively recent, so-called xe2x80x9cdynamic separation controlxe2x80x9d uses perturbations oscillating near the surface, just ahead of the separation point, as are illustrated in U.S. Pat. No. 5,209,438. These include: pivotal flaps which oscillate from being flush with the surface to having a downstream edge thereof extending out from the surface; ribbons parallel to the surface, the mean position of which is oscillated between being within the surface and extending outwardly into the flow; perpendicular obstructions that oscillate in and out of the flow; and rotating vanes (microturbines) that provide periodic obstruction to the flow, and oscillatory blowing. These devices introduce a periodic disturbance in vorticity to the flow, the vortices being amplified in the unstable separating shear layer into large, spanwise vortical structures which convect high momentum flow toward the surface, thereby enabling pressure recovery. Such a flow is neither attached nor separated, under traditional definitions.
However, such perturbations must be actively controlled as a function of all of the flow and geometric parameters, dynamically, requiring expensive modeling of complex unsteady flow structures and/or significant testing to provide information for adapting to flow changes either through open loop scheduling or in response to feedback from sensors in the flow.
A recent variation on the dynamic separation control is the utilization of a so-called xe2x80x9csyntheticxe2x80x9d jet (also referred to as xe2x80x9cacoustic jetxe2x80x9d or xe2x80x9cstreamingxe2x80x9d ) directed perpendicular to the surface upstream of the boundary separation line of the surface. This approach has been reported as being highly parameter dependent, thus also requiring dynamic control; and, the results achieved to date have not been sufficient to merit the cost and complexity thereof.
A totally different boundary layer problem manifests itself in a variety of applications, an important one of which is the inlet fan of an aircraft turbocompressor. At the tip of the blades, there is a phenomena called blade tip leakage through the clearance between the blade tips and the adjacent wall. The power loss as a consequence of blade tip leakage is significant, but the problem has not been solved in a practical way.
Objects of the invention include improved boundary layer flow, improved deterrence of fluid flow boundary layer separation, reduced noise in fluid flow machinery, increased efficiency of fluid flow machinery, reduced blade tip leakage, boundary layer control which is effective, efficient, having low initial cost and zero operating costs, and boundary layer control which is relatively simple and provides little parasitic impact on the host structures and systems.
This invention is predicated in part on the fact that the outflowing jet stream of an acoustic jet will clear the orifice or nozzle area sufficiently before the onset of negative pressure, which therefore will cause replenishment of gas particles within the jet cavity with particles which are other than those in the emitted jet stream. This invention is also predicated in part on our discovery that an acoustic jet directed tangentially into a boundary layer of a gas flow will produce a net negative flow averaged over time which is generally perpendicular to the surface and a net positive flow averaged over time which is generally parallel to the surface.
According to the present invention, an acoustic jet directed into the boundary layer of fluid flow (such as air) is passively powered by ambient, fluid-borne acoustics. The nozzle/chamber combination of the jet has significant resonance at a band of frequencies including frequencies of significant energy in the noise or other pressure variations within the fluid flow so as to produce a nozzle velocity of sufficient intensity to control the boundary layer. The nozzle of the jet preferably is directed at a low angle of incidence to the boundary layer. In one application of the invention, the acoustic jet is directed at a low angle of incidence in the vicinity of the boundary layer separation point of a diffuser thereby to deter or prevent boundary layer separation. In further accord with the invention, the jet may be located at the entrance to a diffuser to deter or prevent boundary layer separation. In another application, the jet is directed from a shroud toward the tips of an axial fan to reduce the size of the boundary layer and mitigate blade tip/wall inefficiencies.
The negative pressure portion of the acoustic jet cycle creates a flow of low momentum flux gas particles perpendicular to the surface, entering the chamber, thereby removing low momentum flux fluid particles from the approaching boundary layer, such particles having energy imparted thereto through acoustic resonance with high energy pressure components in the gas flow, the energized fluid particles, having higher momentum flux, being injected, preferably essentially tangentially, into the boundary layer, either to provide adequate momentum flux in the boundary layer, thereby to deter the onset of boundary layer separation downstream thereof, or to reduce the size of a boundary layer, such as to thereby mitigate blade tip/wall effects of a fan. Use of the noise energy may result in a quieter system, in some cases, by conversion of acoustic energy into gas flow.
In preferred embodiments, the nozzle is directed at as small an acute angle to the boundary layer as is practicable, referred to herein as xe2x80x9csubstantially tangentialxe2x80x9d; the angle may range from near zero degrees to forty or more degrees. However, the noise driven jet of the invention may be used at other angles.
The invention may be practiced utilizing resonant cavities with fixed walls in which the forcing energy is applied through the neck of the jet. The invention may also be practiced utilizing cavities with flexible walls, where the forcing energy can be applied at a point which is remote from the neck and orifice of the jet; this allows locating the flexible wall adjacent to maximal pressure variations for forcing the jet, while locating the neck and orifice at the point which will provide the maximal desired effect on the boundary layer.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.